In fact you can use the Makefile for GCC from our Build Environment and adapt that (e.g. utilities.mk, devenv.cmd) according to your needs (e.g. to use GHS compiler instead of GCC). Of course, this will work only if you have licensed the Professional Edition as that is deployed in full source code which allows you to compile and link the entire GUI stack by yourself. Please contact Green Hills Software for further support (e.g. how to create libraries).
Hope this helps,